The recent trend toward higher integration in the integrated circuit technology poses a demand for finer feature size patterns. Acid-catalyzed chemically amplified resists are thought promising to this end. The light source used for exposure of these resists is most often high-energy radiation such as electron beam, x-ray, excimer laser, gamma-ray or synchrotron radiation. In particular, the electron beam lithography is of interest as the lithography capable of micropatterning to an extremely fine size of 0.1 micron or less. The EB lithography is also regarded essential for forming a pattern on a photomask which serves as a master for pattern exposure.
This image writing with an electron beam takes a time as compared with the full exposure by photolithography employing a photomask. While sensitivity enhancement is required in order to gain an increased throughput, the mask pattern formation requires more accurate control of line width because the photomask serves as a master during semiconductor manufacture. A tradeoff arises between these problems. In particular, it is difficult to control the in-plane uniformity of line width (CD uniformity) because the CD uniformity can be affected by almost all steps (e.g., resist coating, storage after coating, EB image writing, PEB, development, and etching) involved in the mask fabrication. Then for the advanced mask blanks, it is crucial that the blank be coated with a resist which is substantially insensitive to the process, that is, improved in pre-baking temperature dependence after coating, storage stability after coating, age stability in vacuum after image writing, heating temperature dependence after irradiation, and development dependence.
Known methods for forming a resist pattern while insuring the in-plane uniformity of line width include approaches from process and resist aspects. The improvement-by-process is described in JP-A 2006-228816 and JP-A 2003-68633. The approaches from the resist composition aspect include control of glass transition point of a base polymer as disclosed in JP-A 2005-164633, and inclusion of specific recurring units in a base polymer as disclosed in JP-A 2006-91830.